Conditions in bash scripting (if statements)

Posted on Posted in linux

Hello for everybody, in this post I’ll summarize the most important conditions in bash scripting (if statements).

I hope you find it useful

 1. File-based conditions:

Condition True if Example/explanation
[ -a existingfile ] file ‘existingfile’ exists. if [ -a tmp.tmp ]; then
rm -f tmp.tmp # Make sure we’re not bothered by an old temporary file
fi
[ -b blockspecialfile ] file ‘blockspecialfile’ exists and is block special. Block special files are special kernel files found in /dev, mainly used for ATA devices like hard disks, cd-roms and floppy disks.if [ -b /dev/fd0 ]; then
dd if=floppy.img of=/dev/fd0 # Write an image to a floppy
fi
[ -c characterspecialfile ] file ‘characterspecialfile’ exists and is character special. Character special files are special kernel files found in /dev, used for all kinds of purposes (audio hardware, tty’s, but also /dev/null).if [ -c /dev/dsp ]; then
cat raw.wav > /dev/dsp # This actually works for certain raw wav files
fi
[ -d directory ] file ‘directory’ exists and is a directory. In UNIX-style, directories are a special kind of file.if [ -d ~/.kde ]; then
echo “You seem to be a kde user.”
fi
[ -e existingfile ] file ‘existingfile’ exists. (same as -a, see that entry for an example)
[ -f regularfile ] file ‘regularfile’ exists and is a regular file. A regular file is neither a block or character special file nor a directory.if [ -f ~/.bashrc ]; then
source ~/.bashrc
fi
[ -g sgidfile ] file ‘sgidfile’ exists and is set-group-ID. When the SGID-bit is set on a directory, all files created in that directory will inherit the group of the directory.if [ -g . ]; then
echo “Created files are inheriting the group ‘$(ls -ld . | awk ‘{ print $4 }’)’ from the working directory.”
fi
[ -G fileownedbyeffectivegroup ] file ‘fileownedbyeffectivegroup’ exists and is owned by the effective group ID. The effective group id is the primary group id of the executing user.if [ ! -G file ]; then # An exclamation mark inverts the outcome of the condition following it
chgrp $(id -g) file # Change the group if it’s not the effective one
fi
[ -h symboliclink ] file ‘symboliclink’ exists and is a symbolic link. if [ -h $pathtofile ]; then
pathtofile=$(readlink -e $pathtofile) # Make sure $pathtofile contains the actual file and not a symlink to it
fi
[ -k stickyfile ] file ‘stickyfile’ exists and has its sticky bit set. The sticky bit has got quite a history, but is now used to prevent world-writable directories from having their contents deletable by anyone.if [ ! -k /tmp ]; then # An exclamation mark inverts the outcome of the condition following it
echo “Warning! Anyone can delete and/or rename your files in /tmp!”
fi
[ -L symboliclink ] file ‘symboliclink’ exists and is a symbolic link. (same as -h, see that entry for an example)
[ -N modifiedsincelastread ] file ‘modifiedsincelastread’ exists and was modified after the last read. if [ -N /etc/crontab ]; then
killall -HUP crond # SIGHUP makes crond reread all crontabs
fi
[ -O fileownedbyeffectiveuser ] file ‘fileownedbyeffectiveuser’ exists and is owned by the user executing the script. if [ -O file ]; then
chmod 600 file # Makes the file private, which is a bad idea if you don’t own it
fi
[ -p namedpipe ] file ‘namedpipe’ exists and is a named pipe. A named pipe is a file in /dev/fd/ that can be read just once. for a case in which it’s used.if [ -p $file ]; then
cp $file tmp.tmp # Make sure we’ll be able to read
file=”tmp.tmp”    # the file as many times as we like
fi
[ -r readablefile ] file ‘readablefile’ exists and is readable to the script. if [-r file ]; then
content=$(cat file) # Set $content to the content of the file
fi
[ -s nonemptyfile ] file ‘nonemptyfile’ exists and has a size of more than 0 bytes. if [ -s logfile ]; then
gzip logfile    # Backup the old logfile
touch logfile # before creating a fresh one.
fi
[ -S socket ] file ‘socket’ exists and is a socket. A socket file is used for inter-process communication, and features an interface similar to a network connection.if [ -S /var/lib/mysql/mysql.sock ]; then
mysql –socket=/var/lib/mysql/mysql.sock # See this MySQL tip
fi
[ -t openterminal ] file descriptor ‘openterminal’ exists and refers to an open terminal. Virtually everything is done using files on Linux/UNIX, and the terminal is no exception.if [ -t /dev/pts/3 ]; then
echo -e “\nHello there. Message from terminal $(tty) to you.” > /dev/pts/3 # Anyone using that terminal will actually see this message!
fi
[ -u suidfile ] file ‘suidfile’ exists and is set-user-ID. Setting the suid-bit on a file causes execution of that file to be done with the credentials of the owner of the file, not of the executing user.if [ -u executable ]; then
echo “Running program executable as user $(ls -l executable | awk ‘{ print $3 }’).”
fi
[ -w writeablefile ] file ‘writeablefile’ exists and is writeable to the script. if [ -w /dev/hda ]; then
grub-install /dev/hda
fi
[ -x executablefile ] file ‘executablefile’ exists and is executable for the script. Note that the execute permission on a directory means that it’s searchable (you can see which files it contains).if [ -x /root ]; then
echo “You can view the contents of the /root directory.”
fi
[ newerfile -nt olderfile ] file ‘newerfile’ was changed more recently than ‘olderfile’, or if ‘newerfile’ exists and ‘olderfile’ doesn’t. if [ story.txt1 -nt story.txt ]; then
echo “story.txt1 is newer than story.txt; I suggest continuing with the former.”
fi
[ olderfile -ot newerfile ] file ‘olderfile’ was changed longer ago than ‘newerfile’, or if ‘newerfile’ exists and ‘olderfile’ doesn’t. if [ /mnt/remote/remotefile -ot localfile ]; then
cp -f localfile /mnt/remote/remotefile # Make sure the remote location has the newest version of the file, too
fi
[ same -ef file ] file ‘same’ and file ‘file’ refer to the same device/inode number. if [ /dev/cdrom -ef /dev/dvd ]; then
echo “Your primary cd drive appears to read dvd’s, too.”
fi

 2. String-based conditions:

Condition True if Example/explanation
[ STRING1 == STRING2 ] STRING1 is equal to STRING2. if [ “$1” == “moo” ]; then
echo $cow # Ever tried executing ‘apt-get moo’?
fiNote: you can also use a single “=” instead of a double one.
[ STRING1 != STRING2 ] STRING1 is not equal to STRING2. if [ “$userinput” != “$password” ]; then
echo “Access denied! Wrong password!”
exit 1 # Stops script execution right here
fi
[ STRING1 \> STRING2 ] STRING1 sorts after STRING2 in the current locale (lexographically). The backslash before the angle bracket is there because the bracket needs to be escaped to be interpreted correctly. As an example we have a basic bubble sort:(Don’t feel ashamed if you don’t understand this, it is a more complex example)array=( linux tutorial blog )
swaps=1
while (( swaps > 0 )); doswaps=0
for (( i=0; i < (( ${#array[@]} – 1 )) ; i++ )); do
if [ “${array[$i]}” \> “${array[$(( i + 1 ))]}” ]; then # Here is the sorting condition
tempstring=${array[$i]}
array[$i]=${array[$(( i + 1 ))]}
array[$(( i + 1 ))]=$tempstring
(( swaps=swaps + 1 ))
fi
done
done
echo ${array[@]} # Returns “blog linux tutorial”
[ STRING1 \< STRING2 ] STRING1 sorts before STRING2 in the current locale (lexographically).
[ -n NONEMPTYSTRING ] NONEMPTYSTRING has a length of more than zero. This condition only accepts valid strings, so be sure to quote anything you give to it.if [ -n “$userinput” ]; then
userinput=parse($userinput) # Only parse if the user actually gave some input.
fiNote that you can also omit the “-n”, as brackets with just a string in it behave the same.
[ -z EMPTYSTRING ] EMPTYSTRING is an empty string. This condition also accepts non-string input, like an uninitialized variable:if [ -z $uninitializedvar ]; then
uninitializedvar=”initialized” # -z returns true on an uninitialized variable, so we initialize it here.
fi
Double-bracket syntax only:
[[ STRING1 =~ REGEXPATTERN ]]
STRING1 matches REGEXPATTERN. If you are familiar with Regular Expressions, you can use this conditions to perform a regex match.if [[ “$email” =~ “\b[A-Za-z0-9._%+-]+@[A-Za-z0-9.-]+\.[A-Za-z]{2,4}\b” ]]; then
echo “\$email contains a valid e-mail address.”
fi

 3. Arithmetic (number-based) conditions:

Condition True if Example/explanation
[ NUM1 -eq NUM2 ] NUM1 is EQual to NUM2. These conditions only accept integer numbers. Strings will be converted to integer numbers, if possible. Some random examples:if [ $? -eq 0 ]; then # $? returns the exit status of the previous command
echo “Previous command ran succesfully.”
fiif [ $(ps -p $pid -o ni=) -ne $(nice) ]; then
echo “Process $pid is running with a non-default nice value”
fiif [ $num -lt 0 ]; then
echo “Negative numbers not allowed; exiting…”
exit 1
fi
[ NUM1 -ne NUM2 ] NUM1 is Not Equal to NUM2.
[ NUM1 -gt NUM2 ] NUM1 is Greater Than NUM2.
[ NUM1 -ge NUM2 ] NUM1 is Greater than or Equal to NUM2.
[ NUM1 -lt NUM2 ] NUM1 is Less Than NUM2.
[ NUM1 -le NUM2 ] NUM1 is Less than or Equal to NUM2.

4. Miscellaneous conditions:

Condition True if Example/explanation
[ -o shelloption ] shell option ‘shelloption’ is enabled. Shell options modify the behaviour of bash, except a few unmodifiable ones that indicate the shell status.if [ ! -o checkwinsize ] # An exclamation mark inverts the outcome of the condition following it
echo “Shell option checkwinsize is disabled; enabling it so you can resize you terminal window without problems.”
shopt -s checkwinsize # This shell option is modifiable
fiif [ -o login_shell ]; then
echo “This a a login shell.” # This shell option is not modifiable

fi

5. Double-parenthesis syntax conditions:

Condition True if Example/explanation
(( NUM1 == NUM2 )) NUM1 is equal to NUM2. These conditions only accept integer numbers. Strings will be converted to integer numbers, if possible. Some random examples:if (( $? == 0 )); then # $? returns the exit status of the previous command
echo “Previous command ran succesfully.”
fiif (( $(ps -p $pid -o ni=) != $(nice) )); then
echo “Process $pid is running with a non-default nice value”
fiif (( $num < 0 )); then
echo “Negative numbers not allowed; exiting…”
exit 1
fi
(( NUM1 != NUM2 )) NUM1 is not equal to NUM2.
(( NUM1 > NUM2 )) NUM1 is greater than NUM2.
(( NUM1 >= NUM2 )) NUM1 is greater than or equal to NUM2.
(( NUM1 < NUM2 )) NUM1 is less than NUM2.
(( NUM1 <= NUM2 )) NUM1 is less than or equal to NUM2.

 

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